The present invention relates to a regenerative optical amplifier having a resonator for amplifying light, and using a polarizing element driven by the application of voltage.
Regenerative amplifiers are amplifying devices that trap pulses of light supplied from a laser light source in a resonator for amplification, and extract the amplified light pulses from the resonator to obtain high-power light pulses. Polarizing elements such as Pockels cells driven by application of voltage are often used in order to trap and extract the light pulses. Pockels cells are polarizing elements that are driven by the application of voltage to crystals exhibiting the Pockels effect, in which the optical properties of the crystal can be made to become anisotropic with the application of voltage, thereby changing the state of polarization of the light passing through the crystal. In regenerative amplifiers making use of Pockels cells, the use of polarizers and the like in combination with Pockels cells allows for optical switching whereby light pulses are trapped in the resonator where the light is amplified, and then extracted.
FIG. 5 is a drawing showing an example of the structure of a conventional regenerative optical amplifier. The illustrated regenerative optical amplifier consists of a polarizer 10, a Pockels cell 11, a reflective mirror 12, a laser crystal (lasing medium) 13 and a reflective mirror 14. A laser light pulse from a laser light source is supplied as input light to the polarizer 10. The polarizer 10 reflects S-polarized light, i.e. light that is linearly polarized in the direction perpendicular to the plane of the drawing, and passes P-polarized light, i.e. light that is linearly polarized in the direction parallel to the plane of the drawing. When an S-polarized laser light pulse is supplied as the input light, the input light is reflected by the polarizer 10 and directed towards the Pockels cell 11. The Pockels cell 11 is a polarizing element that is driven by a voltage VPC, such that when the voltage VPC is applied, the polarization of input light which has once passed through and been turned back by the reflective mirror 12 is rotated by 90 degrees. The input light which passes through the Pockels cell 11 while this voltage VPC is applied forms a P-polarized light pulse that passes through the polarizer 10, after which it continues through the resonator formed by the reflective mirror 12, laser crystal 13 and reflective mirror 14. As a result, the input light which has been transformed into a P-polarized light pulse is injected into the resonator as a seed light pulse to be amplified.
The light pulse trapped in the resonator is amplified in the process of repeatedly passing back and forth through the laser crystal 13 while oscillating in the resonator. The amplified light pulse can be converted into an S-polarized light pulse by applying a voltage VPC similar to the above at the moment it passes from the reflective mirror 12 into the Pockels cell 11. The amplified light pulse which has been S-polarized in this way is reflected at the polarizer 10, thus enabling a high-power amplified light pulse to be extracted as the output. The extracted output beam is separated from the input beam by light separating means, not shown, provided outside the resonator.
In regenerative optical amplifiers using Pockels cells as described above, the timing of the process for trapping and extracting light pulses is determined by the timing by which voltage is applied to the Pockels cells, thus offering the advantage of enabling the sequence of seed light pulses injected into the resonator and the amount of amplification of the injected seed light pulses to be readily controlled. Furthermore, regenerative optical amplifiers using Pockels cells are usually capable of providing amplified light pulses of generally good quality.
For these reasons, numerous types of regenerative optical amplifiers with different configurations of the Pockels cells and modes of application of voltage to the Pockels cells have been proposed. For example, with regard to configurations of the Pockels cells, regenerative optical amplifiers provided with two Pockels cells, one for entrapment and one for extraction of light pulses, have been proposed. In these regenerative optical amplifiers, voltage is applied to one of the Pockels cells only during entrapment of light pulses (during injection of seed light pulses), and voltage is applied to the other Pockels cell only during extraction of the amplified light pulses. Additionally, with regard to modes of application of voltage to Pockels cells, proposals have included those which apply a voltage that rotates the polarization of transmitted light by 45 degrees (to form a P-polarized or S-polarized beam), and those which apply a voltage to trap light pulses, such that the application of voltage is maintained during amplification, and the amplified light pulses are released when the voltage is returned to 0 V.
While Pockels cells change the polarization of transmitted light depending on the applied voltage as described above, they usually require application of a high voltage on the order of several kV in order to achieve the change in polarization required by regenerative optical amplifiers. For example, in a normal Pockels cell of KD*P crystal, about 7 kV must be applied in order to rotate the direction of polarization of laser light of wavelength 1064 nm by 90 degrees. Additionally, light pulses trapped in a resonator by applying voltage to a Pockels cell can make a roundtrip of a resonator and return to the original position at the Pockels cell in a few tens of ns (e.g. if the length of the resonator is 1.5-3 m, then a roundtrip of the resonator will take (roundtrip optical path length of resonator)/(speed of light)=(1.5-3 m)/(3×108 m/s)=10-20 ns). For this reason, the voltage applied to a Pockels cell must be an extremely short pulse which rises in a few ns and drops away again in a few ns. Thus, regenerative optical amplifiers using Pockels cells involve the problem of how to apply a high voltage on the order of kV to the Pockels cells in the short span of a few ns, complicating the drive circuitry (control circuits for the applied voltage) required for these Pockels cells.
For example, a voltage of VPC as shown in the timing chart of FIG. 6 must be applied to the regenerative optical amplifiers of the above-mentioned FIG. 5. In the timing chart of FIG. 6, the top portion shows the change in the applied voltage VPC over time, with the horizontal axis indicating the time and the vertical axis indicating the magnitude of the applied voltage VPC. The bottom portion shows the change in intensity of the optical pulse in the resonator and the output beam over the passage of time, with the horizontal axis indicating the time just as in the top portion, the upper half of the vertical axis indicating the intensity of light transmitted by the Pockels cell 11 (of the light coming from the reflective mirror 12) and the lower half of the vertical axis indicating the intensity of the output beam. The voltage of the pulse at the time interval TP1 is the voltage VPC that is applied at the moment the input beam returns to the Pockels cell 11 as described above. This causes the injected seed light pulse to be amplified over the amplification time TAT, so that the light pulse in the resonator will develop as shown in the upper half of the bottom portion. Then, the voltage VPC is applied as a pulse voltage during the time interval TP2, which is the moment at which the amplified light pulse enters the Pockels cell 11 as described above, so that an S-polarized amplified light pulse appears as the output beam. Therefore, with regard to the regenerative optical amplifier of the FIG. 5, the two pulse voltages shown in the top portion of FIG. 6 must reach values on the order of kV, and the time intervals TP1 and TP2 must both be 10 ns or less.
Furthermore, the amplification time TAT for amplifying the injected seed light pulse is determined based on the time required for the pulse energy of the amplified light pulse to reach saturation when amplified by the laser crystal 13. A time of about 100-200 ns is usually selected. In contrast, the regenerative optical amplifier of FIG. 5 as described above requires two applications of a pulse voltage of kV order lasting just 10 ns or less. Moreover, it is very difficult to achieve symmetry in the rise and drop waveforms of the pulse voltages. Therefore, an amplification time TAT on the order of a few hundred ns must be selected although this is more than is actually required, thereby placing restrictions on the level of control allowed for the amplification time TAT, and making it difficult to control the amplification time TAT with precision.
Recently, there has been a demand to obtain high-speed amplified light pulses of at least a few tens of kHz by repetitive high-speed light amplification with regenerative optical amplifiers. For this reason, not only do the drive circuits of Pockels cells require high-voltage power sources with quickly rising and falling output, but the high-voltage power sources also need to be capable of responding to high-speed repetitive operation on the order of a few tens of kHz, thus making it very difficult in actual practice to achieve a drive circuit for Pockels cells.
On the other hand, the above-described regenerative optical amplifiers having two Pockels cells only require each Pockels cell to be driven once in order to obtain an output beam, so that there is no need for repeated application of voltage to a single Pockels cell within the amplification time interval. However, this structure is no different from the regenerative optical amplifier at least in that a high voltage on the order of kV lasting only a few ns must be applied in order to drive the respective Pockels cells, thus making it difficult to achieve a drive circuit for the Pockels cells.
On the other hand, the types of regenerative optical amplifiers that maintain the voltage applied to the Pockels cell during light amplification require the applied voltage to rise quickly when trapping the light pulse and to drop quickly when extracting the amplified light pulse, thus requiring output properties similar to those described above in the high-voltage power sources to be used as drive circuits. In particular, complicated circuitry is needed to achieve a fast rise time, thus leaving unsolved the problem of how to make a suitable drive circuit for the Pockels cell. Additionally, once the applied voltage has quickly risen, the amplification time will last until the voltage quickly drops away, thus requiring restrictions on the control of the amplification time just as in the regenerative optical amplifier of FIG. 5, and making it difficult to increase the speed of the repetitive operation of the optical amplifier.